Development

COGNITIVE

The ages of birth to ten are a peak period of sensitivity for learning. During much of this time, a child's brain actually consumes twice as much glucose as an adult's. The infant brain doubles in size during the first year of life. At birth, each neuron in the cerebral cortex has around 2,500 synapses. By the age of two to three years, each neuron has 15,000 synapses. This massive growth in connectivity is matched by terrific pruning. As the brain adapts itself to its surroundings and becomes more specialized, old connections are pruned away. This is the main mechanism by which cognitive development fits itself to the social and cultural environment of the child. Yet although the plasticity of the developing child's brain is remarkable, equally remarkable is the similarity in cognitive development that is found across cultures and social contexts.

Piaget's Theory of Cognitive Development

Cognitive-developmental psychology traditionally coped with cross-cultural similarity by positing culture-general theories of knowledge development. The most famous of these theories was that proposed by Jean Piaget. Piaget suggested that reasoning in all kinds of cognitive domains (e.g., moral reasoning, physical reasoning, and logical reasoning) progressed through a series of universal stages that transcended culture and context. For Piaget, children progressed through three levels of knowing or of mental organization (Smith 2002). These were infancy (during which knowledge was based on action—the sensorimotor period), childhood (based on representational thought—the attainment of concrete operations), and adolescence (based on formal understanding—the attainment of formal operations). Piaget stressed that the levels in his theory were levels of knowledge, not levels of the child. He also suggested that the stages were not age-related, although he did provide indicative ages at which they occurred (sensorimotor, birth to two years; preoperational, two to seven years; concrete operations, seven to eleven years; formal operations, adolescence onwards). Nevertheless, he is usually characterized as a stage theoretician, and has been much criticized accordingly. Even quite young children can be shown to possess cognitive abilities that, according to Piaget's stage theory, they should not have at a given stage. For example, three-year-old children can reason by analogy, characterized by Piaget as a formal operation (see Goswami 1998). Other criticisms concern Piaget's assumptions that early thought is not representational, and that language plays a peripheral role in cognitive development.

Vygotsky's Theory of Cognitive Development

Lev Vygotsky differed from Piaget in that the role of social context and culture in children's cognition was a central part of his theory (Rowe and Wertsch 2002). Rather than seeing the development of knowledge as transcending culture and context, Vygotsky argued that an understanding of how knowledge develops requires an understanding of the social and historical origins of knowledge and of changes in that knowledge. He also proposed a central role for language in cognitive development. Vygotsky argued that human knowledge originates in socially meaningful activity and is shaped by language. Processes that originate in the social world are transferred to the inner mental world (inner speech), and shape the development of
higher cognitive processes such as problem-solving. A key part of this transfer lies in the child's mastery of the symbolic or artificial stimuli (signs) characteristic of the child's culture, such as language. Part of the development of children's thinking therefore requires apprenticeship into culturally specific cognitive and social practices. According to Vygotsky, cognitive development does not happen just in the head of the child. Rather, it is a process of learning to operate with physical, symbolic, and cognitive tools in ways that in themselves change cognitive processes. The difference between a child's individual performance and that child's performance when guided by experts is metaphorically described by Vygotsky's zone of proximal development (ZPD). The ZPD was described by Vygotsky (1978) as "the distance between the actual developmental level as determined by independent problem solving and the level of potential development as determined through problem solving under adult guidance or in collaboration with more capable peers" (p. 86). This notion of an enhanced level of mental functioning when an expert guides an apprentice has been influential in education and in the study of learning disability.

Information Processing Theories of Cognitive Development

Later theories of cognitive development have been based on a computer metaphor. The idea that the brain is like a computer, able to take certain inputs, convert them into representations, and use these representations to compute certain outputs, led to new theoretical models for cognitive development called information-processing and connectionist models. Neo-Piagetian information processing theories explained cognitive development in terms of two fundamental components: the child's assumed available memory storage and the level of complexity at which the child was assumed to be capable of processing information (e.g., Case 1992; Halford 1993). Connectionist models are learning systems, and are loosely based on principles of neural information processing. They are intended to employ the same style of computation as the brain (they do not model exactly what is understood about neural circuits and the computational primitives/representations in the cortex and elsewhere in the brain that are extracted from environmental input). Connectionist models have proved particularly useful for their insights into possible causes of atypical development. For example, small changes in learning algorithms (routines) can model either reading development or dyslexia. This suggests that very small differences in a basic aspect of cognitive processing can lead eventually to quite noticeable differences in developmental outcome. Connectionist models also force the theorist to be more aware of the effects of incremental and context-dependent piecemeal learning for the child's development: every input to a connectionist system makes a difference to final learning, and theorists must be aware that every aspect of a child's environment will contribute to cognitive development.

Latest Perspectives on Cognitive Development

According to the latest conceptualizations of cognitive development, the infant begins the process of knowledge acquisition with a set of core principles that guide and constrain future cognitive development (e.g., Gopnik, Meltzoff, and Kuhl 1999; Goswami 2002). These core principles are either innate, or are given by simple perceptual information such as a sensitivity to contingency (events that appear contingent on one another). Experience of the physical and social worlds allows infants to enrich and revise these initial expectations, and even to replace them with new understandings. Knowledge acquisition is guided by the core constraints, and also by the ways in which surrounding adults behave—the social, emotional, and cultural contexts within which learning takes place. The kinds of innate or early-developing core principles postulated include physical principles like solidity and continuity of objects (e.g., that one object can only be in one place at a time) (Spelke et al. 1992), expecting words to refer to commonalities among objects (e.g., words label shared categories, functions, or perceptual aspects of objects) (Waxman 2002), and a basic animate/inanimate distinction (e.g., living versus nonliving (Gelman 1990). In contrast to traditional theories, therefore, current cognitive developmental psychology does not characterize the newborn as incapable of distinguishing self from other, incapable of forming representations, or incapable of retaining memories. Rather, newborns are characterized as active learners, equipped with certain innate expectations that, although quite primitive, enable them to benefit hugely from experience. The extent of this benefit depends on powerful learning mechanisms, such as the absorption of statistical regularities in the environment (e.g., in early perceptual tuning to the sounds of one's native language); making relational mappings, as in mapping the actions of other people onto the actions of one's own body (infant imitation); mapping the responses of another person to one's own emotional states; and explanation-based learning: noticing causal regularities in environmental information and seeking explanations for them, as in noticing that objects sometimes fall unexpectedly, and that this tends to occur when they are insufficiently supported (see Goswami 2002). Following are two examples of how the social, emotional and cultural contexts within which learning takes place affects cognitive development within this newer theoretical framework.

Social Cognition

Infants are innately interested in, and attentive to, people. Even newborn babies can imitate facial expressions, and older infants prefer to imitate people rather than machines (Meltzoff 1995). Joint attention skills develop by about nine months, and infants probably have a basic notion of agency by the end of the first year. Infants' conscious awareness of their own emotional states and of how they are related to the actions of their caregivers also develops during the first year of life. Although an understanding of representational mental states (e.g., thoughts, beliefs, knowledge, ideas, or false beliefs) develops more slowly, a basic understanding of desires and emotions is present relatively early (by around two years). This early focus on other people means that parents and caretakers have an enormously important role to play in cognitive development.

As an illustration, take pretending, an early example of the child's symbolic capacity. Children across the world play pretend games, and pretending is important both for the development of the cognitive understanding of the minds of others (Lillard 2002) and for the development of social cognition more generally. Pretence activities focused on objects and props typically begin during the second year of life, and sociodramatic pretending with caretakers and peers typically emerges at around three to four years. Cultural contexts affect children's choice of pretend play topics. For example, the pretend play of U.S. preschoolers shows greater enactment of fantasy themes than the pretend play of Taiwanese children, whereas Taiwanese children spend a lot more time playing games about social routines and "proper" conduct (Haight et al. 1999). Parental attitudes and parental engagement also affect the frequency of pretend play, with more pretend play found in cultures where it is actively encouraged. Thus parents and caretakers act, usually quite unconsciously, in ways that promote and influence cognitive change.

A second illustration comes from research into children's understanding of mental states (theory of mind). A basic division between the mental (thoughts, ideas, beliefs) and the physical (substantive, objective objects) is present from early in childhood (Wellman 2002). As they seek causes and explanations for the actions of others, children gradually develop an understanding of mental states such as beliefs, knowledge, and false beliefs. For example, an understanding of false belief, with a consequent understanding of deception and intentional lying, develops at around four years. One important source of individual differences in the development of theory of mind is parent-child and family relationships. Children with brothers and sisters, particularly those with older siblings, typically show earlier psychological understanding, for example passing false belief tasks at earlier ages than children without siblings (e.g., Youngblade and Dunn 1995). Children whose families openly discuss emotions and feelings also show earlier developments in psychological understanding, particularly if the family discussions analyze the causes of emotions. The ways in which we talk to our children and the things that we talk to them about both play key roles in cognitive development.

The Development of Logical Reasoning

Research into the development of logical reasoning was for a long time dominated by Piaget's idea that development consisted of the child's gradual discovery of formal rules and principles such as transitivity and deductive logic. These formal principles were thought to be domain-general (applying across all fields of learning) and content-independent (applying irrespective of the material concerned), and were assumed to operate in their purest form in totally unfamiliar domains. The existing state of the child's conceptual system was therefore ignored. Late twentieth-century research has demonstrated that difficulties in logical reasoning are not usually determined by the intrinsic logical structure of the task. Rather, they are determined by the content or mode of presentation of the problem itself. This can be shown both across cultures and within different social contexts.

For example, it was believed that young children and adults from less Westernized cultures suffered from an empirical bias in logical (syllogistic) reasoning. If given a classical logical deduction such as "All Kpelle men are rice farmers. Mr. Smith is not a rice farmer. Is he a Kpelle man?", West African Kpelle tribespeople seemed unable to answer correctly (Scribner 1977). They said that they did not know the man in question and thus could not verify whether he was a Kpelle man or not. Young children given similar logical problems showed a similar "empirical bias." They seemed unable to reason about unfamiliar or incongruent information simply by applying deductive logic. However, Maria Dias and Paul Harris (1988; 1990) showed that even preschoolers could reason about incongruent premises if the reasoning task was presented in a "fantasy" mode. When the experimenter pretended that she was on another planet and used a "make-believe" intonation, even four-year-olds could solve syllogisms such as "All cats bark. Rex is a cat. Does Rex bark?" Dias and Harris concluded that young children were capable of deductive reasoning, as long as logical problems were presented in a context that clearly marked for the child that the situation was make-believe.

As another example, take performance on a classic Piagetian task, conservation. The conservation task is a measure of children's understanding of the principle of invariance: quantities do not alter unless something is added or taken away. In the conservation task, a child is shown two identical quantities, such as two rows of five beads arranged in 1:1 correspondence, or two glasses of liquid filled to exactly the same level. An adult experimenter then alters the appearance of one of these quantities while the child is watching. For example, the adult could pour the liquid in one of the glasses into a shorter, wider beaker, or could spread out the beads in one of the rows so that the row looked longer. Piaget showed that in these circumstances, children younger than around seven years told the experimenter that there was now less water in the wider beaker, or that there were more beads in the spread-out row. Again, however, social context plays a role in determining children's performance in this task. For example, when a "naughty teddy" alters the beads in one of the rows instead of an important adult, children as young as four and five years show conservation (McGarrigle and Donaldson 1975). Also, children who grow up in cultures that provide extensive experience with changes in appearance that do not alter quantity show earlier conservation. For example, the children of potters in certain rural societies show very early conservation of mass (Price-Williams, Gordon, and Ramirez 1969). Again, rather than being independent of culture and context, children's logical abilities are to some extent determined by both.

Conclusion

Late twentieth-century theoretical frameworks in cognitive developmental psychology have emphasized the importance of explanation-based learning models of cognitive development. Children are conceptualized as seeking to explain the world around them in terms of the collateral and background information that is available to them. The child's access to such information will vary with individual experience, parental and family practices, educational and cultural practices, and with sociohistorical context. Knowledge acquisition is thought guided by certain core constraints, and also by the ways in which surrounding adults behave—unconsciously transmitting social, emotional, and cultural norms within which learning takes place. The fact that children across the world seem to develop remarkably similar cognitive frameworks suggests that the learning mechanisms in the brain are actually fairly heavily constrained, and that environmental inputs across different cultures and social contexts share considerably more similarities than differences.

youngblade, l. m., and dunn, j. (1995). "individual differences in young children's pretend play with mother and sibling: links to relationships and understanding of other people's feelings and beliefs." child development 66:1472–1492.

usha goswami

EMOTIONAL

Broadly stated, aspects of lifespan emotional development include emotional expression and experience, understanding emotions of self and others, and emotion regulation. As such, emotional development is central to children's ability to interact and form relationships with others. Much of the variation in children's emotional development derives from experiences within the family.

Theories of Emotion

Several perspectives help explain the role of emotion in development. Some theorists emphasize that emotions occur during events involving self and environment, but that events must be cognitively appraised before an emotion is experienced; this appraisal occurs with reference to one's goals (Frijda, Kuipers, and ter Schure 1989; Lazarus 1991). The social constructivist approach (e.g., Saarni 1999) also highlights appraisal, but focuses on emotions as social products based on cultural beliefs. In contrast, Differential Emotions Theory asserts that different emotions are already present at birth (Izard 1991). Keith Oatley and Jennifer Jenkins (1996) assimilate these divergent views, holding that emotions derive from a universal biological core, but also contain an appraisal/semantic component that is largely a product of social construction.

Emotional Competence

Both Susanne Denham (1998) and Carolyn Saarni (1990, 1999) have written about children's emotional competence; they agree that, although there are no overarching stages for emotional development, children become increasingly sophisticated in their expression and experience, understanding, and regulation of emotions. These early foundations of emotional competence contribute to mental health throughout the lifespan.

Socialization of emotional competence. Because emotions are inherently social, skills of emotional competence are vividly played out during interaction and within relationships with others. As noted by Joseph Campos and Karen Barrett (1984), emotions provide useful information for self and others. This entry focuses on the emotional transactions between parent and child and on parents' contributions to emotional competence.

Amy Halberstadt (1991) has highlighted three possible mechanisms of parents' socialization of emotional competence: modeling, reactions to children's emotions, and teaching about emotions. The theories of psychologists like Sylvan Tomkins (1963, 1991), as well as empirical findings from late twentieth-century research (e.g., Denham, Cook and Zoller 1992; Denham and Kochanoff, in press; Denham et al. 1997; Denham, Zoller, and Couchoud 1994; Dunn, Brown, and Beardsall 1991; Eisenberg, Cumberland, and Spinrad 1998; Eisenberg and Fabes 1994; Eisenberg, Fabes, and Murphy 1996; Eisenberg et al. 1999), predict that parents' positive emotional expression and experience, accepting and helpful reactions to children's emotions, and emphasis on teaching about emotions in the family, contribute to young children's more sophisticated emotional competence.

Over and above these mechanisms for the socialization of emotion, cultural issues are paramount (e.g., Kitayama and Markus 1994; Lutz 1994; Markus and Kitayama 1994; Matsumoto 1994; Matsumoto et al. 1988; Saarni 1998; Shiraev and Levy 2001). Parents socialize their children based on specific cultural values and norms, but cross-cultural similarities and differences remain to be delineated. In both Japan and the United States, people often agree on the antecedents and evaluative components of emotional experience, and even on some primitive aspects of appraisal (e.g., "I was scared of the loud noise; that didn't feel good; it seemed certain that something bad was about to happen; I had to decide how to cope"). Nevertheless, they differ markedly on some of the more advanced aspects of appraisal, including control of and responsibility for emotion. In the United States, people might state, "I have to show this emotion," or even "I am not responsible for this emotion," whereas the Japanese might say "I should not show this emotion" and "I am responsible for this emotion" (Mauro, Sato, and Tucker 1992; Nakamura, Buck, and Kenny 1990). Given these differences, the goals of emotion socialization surely differ across the two cultures.

Regarding modeling, children observe parents' ever-present emotions, and incorporate this learning into their expressive behavior. Parents' expressiveness also teaches children which emotions are acceptable in which contexts. Their emotional displays tell children about the emotional significance of differing events, behaviors that may accompany differing emotions, and others' likely reactions. A mostly positive emotional family climate makes learning about emotions accessible to children (e.g., Garner, Jones, and Miner 1994). Thus, parents' expressiveness is associated with children's understanding of emotions as well as their expressiveness (Denham and Grout 1992, 1993; Denham, Zoller, and Couchoud 1994).

Parents may cultivate some emotional expressions, but not others. Western cultures urge children to separate self from others and express themselves, but many non-Western cultures view people as fundamentally connected, with the goal of socialization attunement or alignment of one's actions and reactions with that of others'. Thus, in Japan, the public display of emotions is mostly discouraged because it is seen as disruptive, leading us to expect Japanese parents to model mostly low intensity emotions (Ujiie 1997).

Moreover, there is a qualitative difference in the emotions modeled. Valued emotions accompanying interdependence—friendliness, affiliation, calmness, smoothness, and connectedness—would be most available for observation by Japanese children. In contrast, anger, regarded as extremely negative in Japan because it disturbs interdependence, would be modeled less (Ujiie 1997). Research on these culturally unique aspects of socialization of emotions, however, is still largely lacking.

Parents' contingent reactions to children's emotional displays are also linked to children's emotionally competent expression, experience, understanding, and regulation of emotions (Denham, Zoller, Couchoud 1994; Denham et al. 1997; Eisenberg and Fabes 1994; Eisenberg, Fabes, and Murphy 1996; Eisenberg et al. 1999). Contingent reactions include behavioral and emotional encouragement or discouragement of specific emotions. Parents who dismiss emotions may actively punish children for showing emotions, or they may want to be helpful, but ignore their child's emotions in an effort to "make it better." Children who experience negative reactions are distressed by their parents' reactions as well as the events that originally elicited emotion.

Positive reactions, such as tolerance or comforting, convey a very different message—that emotions are manageable, even useful. Good emotion coaches, at least in the United States, accept children's experiences of emotion and their expression of emotions that do not harm others; they empathize with and validate emotions. In fact, emotional moments may be opportunities for parent-child intimacy (Gottman, Katz, and Hooven 1997).

Japanese parents' reactions to children's emotions differ from U.S. parents', although not at every age or in every situation. In general, U.S. parents
see expression of emotions as legitimate and part of healthy self-assertion. Japanese mothers also respond positively to their infants and young children's emotions (Kanaya, Nakamura, and Miyake 1989), but gradually emphasize, more than U.S. parents, parenting goals of inhibitory self-regulation and acquisition of good manners. Thus, for children older than about three years, Japanese mothers react most positively to children's suppression of emotion and demonstration of empathy. Compared to U.S. parents, they especially discourage negative emotional expression (Kojima 2000).

Socializers' tendencies to discuss emotions, if nested within a warm parent-child relationship, assist the child acquiring emotional competence (Kochanoff 2001). Parents directly teach their children about emotions, explaining its relation to an observed event or expression, directing attention to salient emotional cues, and helping children understand and manage their own responses.

Parents who are aware of emotions and talk about them in a differentiated manner (e.g., clarifying and explaining, rather than "preaching") assist their children in experiencing and regulating their own emotions. Children of such parents gradually formulate a coherent body of knowledge about emotional expressions, situations, and causes (Denham, Cook, and Zoller 1992; Denham, Zoller, and Couchoud 1994).

Late twentieth-century research suggests that Japanese mothers also talk to their preschoolers about emotions (e.g., Clancy, 1999; Sonoda and Muto 1996). They use emotion language for similar reasons as U.S. mothers; what differs is the content of their conversations, which focus on aspects of emotion relevant for Japanese culture.

Thus, positive elements of emotion socialization seem clear. Moreover, there is some evidence that parents' support of one another also helps to ensure such positive elements (Denham and Kochanoff, in press). However, do beneficial aspects of parents' socialization of emotion differ across children's ages, or across parents? Although more research is needed in this area, it is predicted that these socializing techniques would occur across development and parents, albeit with different emphasized emotions, and different aspects yielding positive child outcomes. In part, however, these questions require an elucidation of children's changing skills of emotional competence.

Expression and experience of emotions. An important element of emotional competence is emotional expressiveness, the sending of affective messages. Emotions must be expressed in keeping with the child's goals, and in accordance with the social context; goals of self and others must be coordinated. Thus, emotional competence includes expressing emotions in a way that is advantageous to moment-to-moment interaction and relationships over time (Halberstadt, Denham, and Dunsmore 2001).

First, emotionally competent individuals are aware that an affective message needs to be sent in a given context. But what affective message should be sent, for interaction to proceed smoothly? Children slowly learn which expressions of emotion facilitate specific goals. Second, children also come to determine the appropriate affective message, one that works in the setting or with a specific playmate. Third, children must also learn how to send the affective message convincingly. Method, intensity, and timing of an affective message are crucial to its meaning, and eventual success or failure.

After preschool, children learn that their goals are not always met by freely showing their most intense feelings. For example, grade-schoolers regulate anger in anticipation of the negative consequences they expect in specific situations or from specific persons (e.g., Zeman and Shipman 1996). Along with the cool rule mandating more muted emotions within most social settings, older children's emotional messages become more complex, with the use of more blended signals, and betterdifferentiated expressions of social emotions.

These general tenets of competent experience and expression of emotion may be universal, but children from different cultures differ in the emotions they express. For example, Japanese preschoolers show less anger and distress in conflict situations than U.S. children, even though the two groups' prosocial and conflict behaviors do not differ (Zahn-Waxler et al. 1996). These differences fit the Japanese taboo on publicly displayed negative emotions.

Japanese toddlers and preschoolers' expressiveness is not always different from that of their U.S. peers'. For example, they express empathy in response to others' distress. Even this similarity, however, may arise from differing cultural imperatives. Japanese youngsters are encouraged to feel as one with their group, whereas Western children are encouraged to feel the state of another as part of their increasingly autonomous regulation of emotional states. The import of these subtle differences needs further exploration.

Understanding emotions. Emotion knowledge predicts later social functioning, such as social acceptance by peers. By preschool, most children can infer basic emotions from expressions or situations, and understand their consequences. Preschoolers gradually come to differentiate among the negative emotions, and become increasingly capable of using emotional language. Furthermore, young children begin to identify other peoples' emotions, even when they may differ from their own (Denham, 1986; Fabes et al. 1988; Fabes et al. 1991).

Grade-schoolers become more aware of emotional experience, including multiple emotions, and realize that inner and outer emotional states may differ. By middle school, children comprehend the time course of emotions, display rules associated with emotional situations, and moral emotions. They now have an adult-like sense of how different events elicit different emotions in different people, and that enduring personality traits may impact individualized emotional reactions (Gnepp 1989; Olthof, Ferguson, and Luiten 1989).

These general tenets of competent emotion knowledge seem similar for Japanese children. For example, even two-year-olds use some emotion language; by the end of preschool, their understanding of culturally appropriate emotion language is acute (Clancy 1999; Matsuo 1997). They begin to understand dissemblance of emotion (Sawada 1997). As in U.S. research, however, there is a relative dearth of research on older children.

Emotion regulation. Emotion regulation is necessary when the presence or absence of emotional expression and experience interferes with a person's goals. Negative or positive emotions can need regulating, when they threaten to overwhelm or need to be amplified. Children learn to retain or enhance those emotions that are relevant and helpful, to attenuate those that are relevant but not helpful, and to dampen those that are irrelevant. These skills help them to experience a greater sense of well-being and maintain satisfying relationships with others (Thompson 1994).

Early in preschool, much of this emotion regulation is biobehavioral (e.g., thumb sucking), and much is supported by adults. Important cognitive foundations of emotion regulation contribute to the developmental changes observed in emotional competence from preschool to adolescence. Preschoolers gradually begin to use independent coping strategies for emotion regulation, and grade-schoolers refine these strategies—problemsolving, support-seeking, distancing, internalizing, externalizing, distraction, reframing/redefining, cognitive "blunting" (i.e., convincing oneself that one's distress is minimal), and denial.

Older children are uniquely aware of the multiple strategies at their command, and know which are adaptive in specific situations. They also use more cognitive and problem-solving, and fewer support-seeking, strategies. Adolescents appraise the controllability of emotional experiences, shift thoughts intentionally, and reframe situations to reach new solutions (Saarni 1997).

Japanese children, as noted above, are initially very close to their mothers, who assist them in emotion regulation even more than Western mothers. Some researchers have noted, however, that once emotionally distressed, Japanese children find it harder to regain their equilibrium (Kojima 2000). It could be that extended maternal coregulation, coupled with stricter cultural display rules, make it more difficult for these children to self-regulate once distressed. More research is needed to follow up on these findings.

Applications

How can parents become skilled at the emotion socialization techniques appropriate to their culture? In the United States, many intervention programs exist to show parents how to foster children's social-emotional outcomes (Cowan and Cowan 1998). Most focus on parents helping children already showing difficult behavior, delineating remedial steps toward children's self-control and social skills (e.g., Webster-Stratton 1994). Other programs focus on more proactive parenting techniques (e.g., Shure 1993). In none of these programs, however, are emotion socialization techniques central (Greenberg, Kusche, and Mihalic 1998; Olds et al. 1998). Thus, even the best parenting programs generally fail to address emotion socialization directly.

However, parental instruction on emotional competence could be especially promising as a preventive approach. A few programs highlight such techniques—including those of Maurice Elias, Steven Tobias, and Brian Friedlander (1999), John Gottman (1997), and Lawrence Shapiro (1997)— emphasizing the importance of emotion-friendly family climate and parents' specific roles as emotion socializers for young children. Specific attention to the necessity of emotional competence and to the emotion socialization techniques most likely to contribute to it, in families and daycare and schools, is recommended (e.g., Denham and Burton 1996).

Conclusion

Research has delineated considerable information about children's emotional competence and how it is fostered. Nevertheless, much remains to be learned. More detail is necessary about emotional competence, its socialization, and its contribution to social success and well-being, after preschool (O'Neil and Parke 2000). Finally, the field needs to be broadened to include emotional competence and its socialization in non-Western cultures.

kanaya, y.; nakamura, c.; and miyake, k. (1989). "cross-cultural study of expressive behavior of mothers in response to their 5-month-old infants' different emotion expression." research and clinical center for child development 11:25–31.

mauro, r.; sato, k.; and tucker, j. (1992). "the role ofappraisal in human emotions: a cross-cultural study." journal of personality and social psychology 62:301–318.

nakamura, m.; buck, r.; and kenny, d. a. (1990). "relative contributions of expression behavior and contextual information to the judgment of the emotional state of another." journal of personality and social psychology 59:1032–1039.

sonoda, n., and muto, t. (1996). "references to internalstates in mother-child interactions: effect of different settings and maternal individual differences." japanese journal of developmental psychology 7:159–169.

thompson, r. a. (1994). "emotional regulation: a theme in search of definition." in the development of emotion regulation: biological and behavioral considerations, ed. n. a. fox. monographs of the society for research in child development, no. 59.

MORAL

Moral development is a topic of great interest to psychology, philosophy, sociology, and education. How does an infant—born without moral principles—gradually become a person who respects others and can live in society? This question is studied in the context of socialization.

Earlier Theoretical Models: Psychoanalysis and Behaviorism

Theories have approached morality differently. Sigmund Freud (1856–1939) described Oedipus complex to explain the origins of moral conscience, called the superego. The Oedipus complex occurs when a child loves the opposite sex parent and, in order to avoid the anxiety and fear of punishment that this causes, the child identifies with the same sex parent. The child incorporates the same sex parent's prohibitions, starting with "Do not love (sexually) your parent."

For behaviorist theorists, such as Robert R. Sears, Robert Grinder, and Albert Bandura (1982), conscience or morality was considered analogous to the phenomenon of resistance to extinction. H. Hartshorne and M. A. May, at the end of the 1920s, were pioneers in this line of research. Later, Robert Sears, Eleanor Maccoby, and Harry Levin (1957) and other researchers studied the influence of maternal and paternal disciplines upon development of the conscience. These studies found that warm and affectionate parents, who reason with their children rather than punish them physically, are more successful in having their children assimilate the moral values of the culture. Cognitive behaviorists have added other dimensions to this process, such as expectancies (what the child expects is going to happen), incentive value (how much the child wants something), hypothesis testing ("If I do this, then that will happen"), and self-efficacy (one's capacity and confidence on doing something) (Bandura 1977, 1978). In the psychoanalytic and behaviorist models, morality seems to be something that comes from outside, from society, which is internalized.

Cognitive Models

Jean Piaget (1896–1980) and Lawrence Kohlberg's (1928–1987) theories considered the role of the human being as agent in the moral process. These scholars focused on moral judgment: on the knowledge of right and wrong. In the latter decades of the twentieth century, the cognitive approach took over the study of morality, with few studies conducted on moral behavior or feelings. Both Piaget and Kohlberg were influenced by the philosopher Immanuel Kant (1724–1804) and by sociologist Emile Durkheim (1858–1917). From Kant, with the notion of categorical imperative came the idea of universal moral principles, and from Durkheim came the importance of social and collective factors.

Piaget's model. In Piaget's constructivist perspective, he speaks of the interaction between cognitive structures, or stages of development, which are biologically determined, and environmental stimulation. He is most famous for his work on with the identification of universal stages through which thinking evolves in an invariant sequence (i.e., in the same order for all persons of all cultures) (Piaget and Inhelder, 1967).

In The Moral Judgment of the Child (1932),Piaget argues that moral judgment evolves through stages that are roughly parallel to the stages of cognitive development. He observed children behavior and attitudes in games of marbles. He identified stages in the development of rules, and the children's attitudes regarding rules. The first stage consists of sensorimotor (sense organs and motor development) exercises: the child plays with the marbles, for example, with no notion of rules. In this stage, vision and touch are practiced. In the second stage, called egocentric: the child follows his/her own rules, while trying to imitate others' rules. Paradoxically, the child has great respect for the rules, says they cannot be changed, but does not follow them. If one asks four- or five-year-olds, for example, who created the rules of the game of marbles, they might say God, Santa Claus, or "my father," all other authority figures. During this stage the child considers material losses as more serious than intentions. Piaget used pairs of short stories to test this, for example: Peter rushed into the kitchen and accidentally broke twelve glasses that were on a tray behind the door. Johnny got mad because his mother did not let him play outside, picked up a glass on a tray, and threw it on the floor in order to break it. Which of the two boys deserve more punishment, Peter or Johnny? The younger child says it is worse for a child to break a dozen glasses accidentally than one glass on purpose, because twelve glasses will cost more to replace than one glass. In the third stage, beginning cooperation, the child begins to cooperate, follow rules, and understand the importance of intentionality. It is only in the fourth stage, however, that the child is able to codify rules and understand that game rules are arbitrary and can be changed if all players agree beforehand. Conceptions about justice also evolve from retribution and vengeance (in the young
child) to the notion of reform of the culprit and reparation, or making up for wrong doing (in the older child). Immanent justice (punishment by nature) also diminishes. Heteronomy (norms imposed by external forces) is substituted by autonomy (making decisions depending on one's own conscience).

Kohlberg's cognitive model. Lawrence Kohlberg (1958) based his theories on Piaget's ideas. Unlike Piaget, however, Kohlberg presents a more precise conceptualization and discrimination of the stages, and the dimension of heteronomy-autonomy that underlie the stages. His method allows for quantified scores of maturity of moral judgment. The six stages proposed by Kohlberg are subsumed in three levels: preconventional (stages one and two), conventional (stages three and four), and postconventional (stages five and six). In order to understand the meaning of the stages, it is important first to understand the meaning of levels.

The preconventional level is characteristic of younger children, some adolescents, and many criminals. There is not yet any sense of real morality, or any internalization of values. The conventional level is typical of the majority of adolescents and adults in U.S. society (Colby and Kohlberg 1984), and probably all Western societies and even non-Western societies as well (Snarey 1985). At the postconventional level individuals have come to question the morality of the status quo and are able to change laws and cultural rules. Approximately 5 percent of adults reach the postconventional level, usually after age twenty or twenty-five. At stage one, the orientation is toward punishment and obedience; at stage two, morality is geared toward pleasure and satisfaction of one's own needs; at stage three, morality centers on pleasing others and fulfilling conventional roles; at stage four, the emphasis is on law and order; at stage five, the person tries to change unfair laws through democratic channels; and at stage six individual conscience prevails.

Kohlberg interviewed children and adolescents of ages ten, thirteen, and sixteen years, and identified levels and stages of moral development, proposing moral dilemmas such as one about a husband who steals medicine to save his dying wife when all efforts to get money to pay for the expensive drug failed. Another dilemma has to do with a boy who wanted to go camping, and his father promises he may go if he saves money from his newspaper delivery job. Then the father requests the money for himself, in order to go on a fishing trip. Answers to dilemmas are analyzed and the researcher classifies a person's response into one of the six stages. It is not the content (to steal or not to steal, or to give the money to the father or not to give it) that determines a person's stage of moral judgment, but the reasoning behind it. If a person says the husband should not steal it because he could be caught and go to jail, this person is responding at stage one. If one says he should steal the money to look good before his friends, or only if he loves his wife, this person would be responding at stage three. If, in response to the first dilemma, one says stealing is against the law, so the husband should not steal the money, this person is responding at stage four. Valuing human life over the pharmacist's profit situates the respondent at stage five or six. In the second dilemma, appeals to the father's authority and the duty of a son to obey him places a response at stage one, whereas speaking of the importance of fulfilling a promise places a person at a higher stage. Details about the scoring procedure appear in the Manual for scoring the Kohlberg Moral Judgment Interview, which is a guide to evaluating at which stage of moral development a person's responses are at (Colby and Kohlberg 1984).

Many moral education programs in schools are based on Kohlberg's theory, consisting of group discussion of moral dilemmas, as initially proposed by Moshe Blatt and Kohlberg (1975). These debates or discussions create cognitive conflict when a participant is faced with someone's responses, which may be in a higher stage than his/her own. This usually increases level of moral maturity. Kohlberg started involving whole schools, including any teachers, students, or faculty that wanted to participate, in discussing real-life moral dilemmas of the participants' school situation, a technique referred to as just community, which has been proven very valuable (Power, Higgins, and Kohlberg 1989).

Kohlberg claims that there is a core of moral values that are universal, in other words, the sequence of stages is invariant, and the same for every person of each culture. As a result, certain moral values, such as the respect for human life, and not causing harm to others, are upheld in all cultures. John Snarey's (1985) review of the literature supports this notion. He analyzed more that forty studies conducted in twenty-seven different cultures, which support Kohlberg's claim for universality, although the higher stages (five and six) did not appear in all cultures. However, Richard Shweder and his colleagues (1991) argue for the role of culture, based on their research in India (Shweder, Mahapatra, and Miller 1987): they did not find distinctions between conventional and moral transgressions. Jonathan Haidt, Silvia Koller, and Maria da Graça Dias (1993) corroborated those findings in their research with Brazilian children. Contrary to this relativistic view of morality, some neo-Kohlbergians, such as Elliot Turiel (1983) and Larry Nucci (1981), distinguish between moral and conventional domains, and present evidence that even preschool children distinguish between the severity of transgressions of each domain. Carol Gilligan (1982) argues that women's morality is different from, but not inferior to, male morality. Women emphasize the justice of care, whereas males stress justice, which is the central concept in Kohlberg's theory.

Prosocial Behavior

Comparatively few researchers have examined similarities and differences in the positive sides of morality. There have been few examinations of the dilemmas in which one person's needs or desires conflict with those others in need in a context in which the role of prohibitions (e.g., formal laws or rules), authorities' dictates, and formal obligations. However, children and adolescents often are faced with the decision to help others at cost to themselves. Those decisions have been the focus of prosocial moral reasoning research that emphasizes reasoning about moral dilemmas in which one's needs or desires conflict with those of others in need (Eisenberg 1986).

The development of prosocial moral reasoning is consistent with Kohlbergian justice-oriented developmental stages. The similarity is due to the role of cognition as a necessary, but not sufficient factor, for reasoning about moral dilemmas. Nancy Eisenberg and her colleagues (Eisenberg et al. 1995; Eisenberg et al. 1991) have found a developmental progression from hedonistic and needsoriented, to approval-oriented and stereotypic (norm-related), to, finally, empathic and internalized, modes of prosocial moral reasoning during childhood and adolescence. However, in contrast to prohibition-oriented moral reasoning, older children and adolescents express both cognitively sophisticated types of prosocial moral reasoning as well as the less sophisticated types (Eisenberg et al. 1995). Based on socialization theory (Gilligan 1982; Maccoby and Jacklin 1974), individual and group (e.g., cultural, national, and gender) differences in prosocial moral reasoning may be most evident in late adolescence when differences in moral reasoning due to cognitive development are reduced, and socialization processes are consolidated. Thus, by late adolescence (e.g., for college students), prior and current educational experiences, and cultural socialization processes are expected to become increasingly important to individuals' reasoning in moral situations. Consistent with cognitive developmental theory, researchers frequently have found that the sophistication of moral judgment increases during adolescence, presumably due in part to an increase in perspective taking and reflective abstraction skills (Colby et al. 1983; Eisenberg 1986; Rest 1983; Selman 1980).

The processes involved in prosocial moral reasoning and in prosocial behavior (as reported by Carlo et al. 1996; and Eisenberg, Zhou, and Koller 2001) appear to be similar for children and adolescents of different cultures (North American middle-class adolescents compared to low and high socioeconomic status Brazilian adolescents).

For most people, life is continual change: moral character changes as cognitive and emotional developmental processes (from hedonistic or egocentric behaviors to self-reflexive perspective taking and internalized norm-related or other-related judgments and behaviors) combine and as individuals face new social and familial roles and contexts (Mason and Gibbs 1993; Rest and Narvaez 1991). There are increases in personal and social responsibilities that parallel the developmental changes that occur during the life cycle. Each change provides new opportunities for having a greater impact on personal development, society, and others. Although the aforementioned changes are common to many people during the life cycle, ecological theorists (e.g., Bronfenbrenner 1979) suggest that different culture-specific socialization experiences lead to specific developmental outcomes. Socialization experiences, including social norms, expectations, and educational experiences, may indeed be different for individuals from different cultures depending on the beliefs, attitudes, and behaviors deemed desirable for success in that society. These culture-specific experiences may lead to different patterns of thinking about moral and prosocial issues.

turiel, e. (1983). the development of social knowledge.cambridge, uk: cambridge university press.

silvia h. koller
angela m. b. biaggio

SELF

Children's self-evaluations fall into two categories: evaluations of their competence or adequacy in particular life domains (for example, scholastic competence, physical appearance), and evaluations of their overall worth as a person, which is referred to in this entry as self-esteem. An analysis of the effects of parental variables on children's self-evaluations and personality development is timely given claims (see Harris 1998) that parents have little influence on their children's psychological development other than their genetic contribution. There is considerable research to the contrary, which is not to negate the role of genetics. What is needed is a balanced perspective on the nature-nurture controversy, namely, an appreciation of both genetic contributions and the critical role of parent-child interactions beginning in infancy and continuing through adolescence and beyond. Two theories have dominated the study of the effects of parent-child interactions on children's self-representations: William James' (1890) formulation on the determinants of one's level of global self-esteem—that is, the overall evaluation of one's worth as a person (see Harter 1999a)—and Charles Horton Cooley's (1902) theory of the looking-glass self.

James' Theory of the Determinants of Self-Esteem

For James (1890), self-esteem results not from a summary evaluation of one's successes or failures, but rather from an assessment of one's sense of adequacy or competence in areas one deems important. For children, such domains include scholastic competence, athletic competence, peer likability, physical appearance, and behavioral conduct. Thus, if children feel adequate in those domains judged important and are able to discount the importance of domains in which they feel that they have limitations, then they will have high global self-esteem. Those who continue to assign importance to areas in which they perceive weaknesses will report low self-esteem.

The primary contribution of parents to this process lies in the origins of children's judgments of importance. Children naturally come to accept their parents' definition of the importance of success in given domains, particularly in early childhood where parental values and authority are highly respected. Parents who give high importance to academic success, for example, will convey this attitude, directly or indirectly, and their children will come to view the academic arena as extremely important. However, if a particular child has a palpable weakness (for example, a learning disability, a low IQ, or temperamental traits that interfere with the ability to attend to and concentrate on schoolwork), then this child will not be successful. This, in turn creates a discrepancy between high importance and low success, the very formula that leads to low self-esteem from a Jamesian perspective. Conversely, if the child's abilities and talents are convergent with parental values, then there will not be a discrepancy between importance values and the child's success in various domains, and the child will have high self-esteem. Thus, parents' values can directly affect the self-esteem of their children.

Cooley's Theory of the Looking-Glass Self

For Cooley (1902), significant others, notably parents in childhood, constitute social mirrors into which a child gazes to detect parental opinions of the self. These opinions of others are then, in turn, incorporated into a child's sense of self, namely an evaluation of his or her worth as a person. Thus, if parents approve of the self, these positive attitudes are adopted in the form of high self-esteem as well as a sense of adequacy in the specific areas where there is parental feedback (e.g., scholastic competence, athletic competence, behavioral conduct, appearance). Conversely, if parents manifest their disapproval of child's worth or capabilities, the child will devalue the self and experience low self-esteem. Thus, for Cooley, the self is very much a social construction. Numerous studies have documented the fact that approval from significant others is a powerful contributor to a child's sense of self (Harter 1999a).

The manner in which approval from parents is communicated to children is more complex than mere direct verbal feedback (see Harter 1999a). Negative parental opinions can be communicated through a lack of positive feedback. Another family member may also serve as a source of information about parental appraisals. In addition, through observing how parents evaluate others (e.g., siblings), children can gain indirect information about how parents evaluate the self. Thus, if a sibling receives praise but the target child does not, negative self-evaluations can result.

The looking-glass self represents a dynamic process that occurs over the formative years of development. Ideally, children will come to internalize positive approval such that ultimately they are no longer totally dependent upon the opinions of others. That is, they become able to evaluate their own worth, successes, and failures in the absence of either direct feedback or indirect communication. However, there are potential liabilities when the self is developed in the crucible of family interactions (see Harter 1999b). The first and most obvious are liabilities associated with the internalization of unfavorable evaluations of the self by others. The incorporation of disapproving opinions of parents will lead, in turn, to perceptions of personal inadequacy and low self-esteem.

There are liabilities associated with the failure to internalize standards and evaluative judgments of parents, standards and judgments that one should come to own and that can serve as the basis for one's sense of self-worth and as guides in regulating one's behavior. If one is constantly drawn to the social looking glass, if one persists in primarily basing one's sense of self-worth on the appraisals of others, a constellation of negative correlates will arise. Research by Susan Harter, Clare Stocker, and Nancy Robinson (1996) revealed several related liabilities among young adolescents who, rather than internalizing parental opinions of the self, continued to base their self-esteem on the external views of others. First, these adolescents reported significantly greater preoccupation with approval of peers than did those who had internalized the opinions of others. Second, teachers' ratings confirmed the researchers' expectations that those still gazing into the social mirror were more socially distracted in the classroom, devoting less energy to their scholastic activities, given their greater preoccupation with peer approval. Third, the adolescents in the study reported more perceived fluctuations in peer approval. Fourth, they reported greater fluctuations in self-esteem, which is understandable since by definition they were basing their self-esteem on the perceived approval of others. Fifth, they reported lower levels of peer approval, perhaps because in their preoccupation with approval, they engaged in behaviors that did not garner this type of peer support. They may have tried too hard to obtain peer approval or may have employed inappropriate strategies, and in so doing may have annoyed or alienated their classmates. Finally, given that these adolescents, who by definition based their self-esteem on approval, reported lower peer approval, they reported lower self-esteem.

Aspects of early parent-child interactions may prevent the internalization process from developing. If children receive inconsistent feedback—for example, fluctuations between approval and disapproval from parents—it may be difficult for them to internalize a coherent evaluation of the self. Alternatively, receiving support that is conditional upon meeting unrealistic demands of parents may also prevent the internalization of feelings of self-approval. Conditionality can be contrasted to unconditional positive regard (Rogers 1951) in which parents provide general approval for their child. Adolescents do not find conditional support to be personally supportive (Harter 1999a). Rather, it identifies contingencies (e.g., "If you are successful, I will approve of you," "If you do exactly as I say, I will love you"). Thus, an early history of such conditional approval, as well as fluctuating feedback, does not provide the kind of validating support that can be internalized as approval of the self, nor does it provide a consistent pattern of disapproval that can be internalized as lack of acceptance of the self.

An Attachment Theory Perspective on the Self

From an attachment theory perspective, representations and evaluations of oneself can only be considered with the context of the caregiver-child relationship. Thus, as John Bowlby (1969) contended, children who experiences parents as emotionally available, loving, and supportive of their mastery efforts will construct a working model of the self as lovable and competent. In contrast, children who experience attachment figures as rejecting, emotionally unavailable, insensitive and nonsupportive, or inconsistent will construct a working model of the self that is unlovable, incompetent, and generally unworthy (see Bretherton 1991; Sroufe 1990; Verschureren, Buyck, and Marcoen 2001). In addition, those who are securely attached will report more realistic or balanced self-concepts, reporting on both positive and negative characteristics, although typically more positive attributes are cited (see Cassidy 1990; Easterbrooks & Abeles, 2000). That is, securely attached children have more access to both positive and negative self-attributes than do insecurely attached children, who often present an unrealistically positive account of their strengths in an attempt to mask underlying feelings of unworthiness.

Moreover, Lisa Kiang (2001) found that childhood attachment has a long-term effect on self-esteem in the college years. Kiang found that one type of insecure attachment (avoidance) led to the psychological correlates of eating disordered behavior (feelings of ineffectiveness, perfectionism, interpersonal distrust, maturity fears). These psychological symptoms, in turn, took their toll on self-esteem. Thus, patterns of early parent-child interactions can have far-reaching implications for later development, including maladaptive eating practices and low self-esteem.

Cross-Cultural Issues

Attention has shifted to whether attachment dynamics are universal across cultures or are more culture-specific (see Rothbaum, Weisz, Pott, Miyake, and Morelli 2000; Van Ijzendoorn and Sagi 1999). The most thoughtful conclusion is that for evolutionary reasons, the attachment system does have universal characteristics that are designed for infants' survival during a period when they are vulnerable and therefore highly dependent upon parents. That said, how parental sensitivity is specifically defined should also logically vary from culture to culture, depending upon societal values. Whatever these variations, sensitive parenting should lead to securely-attached behavior that, in turn, should lead to positive self-evaluations in culturally-relevant domains as well as to positive self-esteem. However, cultural variations dictate that one develop different instruments to assess self-evaluations in different cultures.

For example, it is noteworthy that those who have taken Western self-perception measures to various Asian cultures (Korea, China, Taiwan, Hong Kong, Japan) have found that the item content may not be relevant and that the question format, which pulls for social comparison, is not appropriate given that social comparison is frowned upon (see Harter 1999a). Thus, researchers need to take a more culture-specific look at what the self means in different cultures, how salient or important it is in different cultures, and with what outcomes it may be associated.

Conclusion

Parent-child behavior within the context of the family has a profound effect on numerous aspects of self-development. Various parental behaviors influence the level of a child's self esteem, domain-specific self-concepts, accuracy of self-evaluations, and preoccupation with approval which can have debilitating effects on the self. Each of these, in turn, has mental health implications since children's self-perceptions are highly related to their mood, namely the extent to which they are cheerful or depressed (see Harter 1999a). Any thoughtful approach to issues involving the self will require a sensitive inquiry into cross-cultural issues. As the world becomes more interconnected and more global, sensitivity to cultural differences and similarities is integral to the understanding of self-development.

Bibliography

bretherton, i. (1991). "pouring new wine into old bottles: the social self as internal working model." in self processes and development: the minnesota symposia on child development, vol. 23, ed. m. r. gunnar and l. a. sroufe. hillsdale, nj: erlbaum.

cassidy, j. (1990). "theoretical and methodological considerations in the study of attachment and the self in young children." in attachment in the preschool years: theory, research, and intervention, ed. m. t. greenberg, d. cicchetti, and e. m. cummings. chicago: university of chicago press.

harter, s.; stocker, c.; and robinson, n. (1996). "the perceived directionality of the link between approval and self-worth: the liabilities of a looking glass self orientation among young adolescents." journal of research on adolescence 6:285–308.

Development

New Dictionary of the History of Ideas
COPYRIGHT 2005 The Gale Group, Inc.

DEVELOPMENT.

Development in biology refers to the process of growth and differentiation that is characteristic of living organisms. It describes the continuous changes during the life cycle of individual organisms from the early stage of a single cell until death. Development also refers to what is today known as the process of evolution, the transformation of species through time. Other meanings of development are connected to economic and psychological processes. The German term Entwicklung has the same connotations, especially with respect to the two temporal processes of ontogeny (individual development) and phylogeny (evolutionary development), and its meaning also extends into artistic and literary domains (Entwicklungsroman ).

Due to the gradual nature of developmental processes and the wide-ranging diversity of organisms (animals, plants, microbes) and modes of reproduction (sexual, asexual), it is not possible to clearly define a unique starting point of development that applies universally to all organisms. Nevertheless, development is a fundamental property of all organisms and one that sets them apart from other physical and chemical systems. In the language of molecular biology, development is the process that translates the sum of the genetic characteristics of an organism (its genotype) into the morphological, physiological, and behavioral features of an individual (its phenotype). Since the 1970s the prevailing interpretation of this process had become increasingly preformistic—the idea that the genotype largely determines the phenotype. With the twenty-first century, however, this view has gradually been replaced by a more interactive, or epigenetic, interpretation of development that sees the individual phenotype as the product of a dynamic interaction between the genotype and the various environments of an organism (cellular, organismal, physical, cultural). These recent positions in developmental biology
also reflect the long-standing dichotomy of interpretations of development—preformistic and epigenetic—that characterized the scientific and philosophical discussion of the last 2,500 years.

The (human) life cycle and several aspects of development, such as the fertilization of plants, the grafting of fruit trees, and the principle of generation, were already known in antiquity. As with so many other areas of knowledge, it was Aristotle who summarized existing knowledge and by adding his own observations created the first inclusive theory of development. Aristotle expressed his conception of development in De generatione animalium (The generation of animals) and in his whole corpus of zoological writings, and development played an integral part in his overall science and philosophy. Aristotle's view of the world was intrinsically dynamic, based on matter and change. Matter is always structured. Form is the realized potential of matter, its entelechy, which is already present within it. In organisms, according to Aristotle, the potential form (entelechy) is gradually realized in the course of development. This dynamic process of development, as well as the resulting organism, requires all four causes of the Aristotelian physics: the material, formal, efficient, and final causes. In embryological development, the female fluid, the menstrual blood, contributes the material cause on which the semen acts, providing the initial stimulus for the dynamic sequence of development. In the course of development the combination of male and female fluids allows the formal and efficient causes to shape the emerging potential of the organism, its telos. This entelechy of the organism, however, has been present from the very beginning as the potential of this particular form of matter (the combination of male and female fluids). In later periods the Aristotelian entelechy has often been identified with the notion of a "soul," but for Aristotle entelechy is not something separate that directs development from the outside, but rather is always already present within the emerging organism as its potential to be realized gradually.

Ideas of Development in the Seventeenth and Eighteenth Centuries: Preformism and Epigenesis

Aristotle's conception of development was shaped by what he could observe—fluids and semen at conception and the gradual emergence of form in the course of development. It is therefore only logical that the next major changes in the philosophical and scientific analysis of development are connected with emerging possibilities of observation during the seventeenth century. One instrument, in particular, played a central role in discussions about development—the microscope. The microscope allowed for the first time analysis of the constitution of those observable fluids at the beginning of development. Looking at semen with his single-lens microscope, Anton van Leeuwenhoek could see structures in the head of the spermatozoa. But what did those structures represent? In the wake of the scientific revolution, a mechanical approach dominated the sciences and medicine. William Harvey had found a mechanical solution to the circulation of blood, and generations of anatomists had analyzed the form and function of the human body in similar terms. In this context of mechanical ideas, Leeuwenhoek's observations took on a specific meaning. For some of his contemporaries, the structures inside the sperm thus represented a smaller, already preformed version of the adult organism, called an "homonculus" by some. Development then was simply a mechanical unfolding and subsequent growth of structures already present at the very beginning in either the sperm or the egg. Others, such as Harvey, continued to advocate the epigenetic position of Aristotle. These epigenesists also claimed that their views only described what they could observe.

Clearly, observations were ambiguous and often fit theory-driven expectations. Preformists were committed to a mechanical and materialistic explanation. They did not want to rely on any form of entelechy or vital force in order to account for development and were also opposed to ideas and reports of spontaneous generation. Epigenesists, on the other hand, were committed to the action of a vital force in nature. They also emphasized the role of observation and pointed out that several facts, such as the existence of hybrids or "monsters," could not easily be explained within the preformist framework. The influence of metaphysical commitments in shaping the interpretation of observations can best be seen in the mid-eighteenth-century debate between Caspar Friedrich Wolf and Charles Bonnet. Both looked at chick embryos at the same stage (twenty-eight hours after fertilization). Both described in detail what they saw—no clearly defined beating heart, for instance. And both arrived at radically different conclusions. For Wolf it was obvious that the heart would only form later due to the agency of a vital force (vis essentialis); Bonnet, on the other hand, concluded that even though he could not see it, the heart must nevertheless already be there.

Metamorphosis and Recapitulation

The eighteenth-century debates about preformism and epigenesis brought development into the spotlight of biological investigations. Ordering the known diversity of life, increasing by the day as a result of European voyages of exploration, was another major concern. For many, especially the Romantic scientists at the turn of the nineteenth century, these two areas of natural history were connected. Did the diversity of nature not arise in the course of development from similar structures? Are the creative principles in nature not the same as in the arts? Pondering these questions on a trip to Italy, the poet-philosopher-scientist Johann Wolfgang von Goethe discovered the principle of metamorphosis and established the foundations of morphology. Specifically, Goethe realized that all the diverse structures of flowering plants are transformations of one basic morphological form, the leaf. Understanding these principles of transformation, or metamorphosis, then allows the scientist or the artist to recreate all existing organic forms, as well as those that could exist but have not yet been realized. This morphological building plan (Bauplan ) is intrinsically dynamic and developmental; it is a principle that unfolds itself in nature small and large, in the individual and the cosmos. Morphogenesis focused, for Goethe and others, on the emergence of form within a context of change.

Ideas about transformation were soon applied to species as well as individuals. In 1809 Jean-Baptiste de Lamarck published
his theory of evolution, which gave development the additional meaning of the transmutation of species. For Lamarck the transmutation of species was driven by an intrinsic drive toward perfection. In this "escalator theory" of evolution, primitive forms, created spontaneously, pass through increasingly complex stages in the course of subsequent generations. The essence of nature is thus transformation, both in the course of individual development and in the generation of the diversity of life. Lamarck's theory was readily attacked, especially by his colleague George Cuvier, the founder of comparative anatomy. Cuvier had established the most sophisticated classification system of animals of his time, based on the recognition of four distinct types of animals and a strict hierarchy of systematic categories within each of these embranchements. Within this system species were considered immutable, and their relationships were defined by the degree of similarity between them.

Development was one way of explaining this similarity among species. All organisms begin their life as fertilized eggs—Karl Ernst von Baer would discover the mammalian egg in 1827—and the early stages of development also resemble each other more closely than later stages. Summarizing these observations, Johann Friedrich Meckel proposed in 1811 that the embryological stages of advanced organisms represent the adult stages of more primitive organisms. This was the first formulation of the principle of recapitulation, in which development became the causal explanation for the similarity as well as the differences between species. The evolutionary implications were obvious. Defending the clear separation of different systematic groups, Karl Ernst von Baer summarized his opposition to the principle of recapitulation in his developmental laws. He stated that no adult organism is like any embryo of another organism, that each developmental trajectory is unique, but that in each developmental sequence the more general features of the organismal structure appear earlier in development, which explains the close similarities between the early embryos of different species.

Von Baer's authority carried the day, but only briefly. In his theory of evolution as descent with modification, Charles Darwin also relied on embryological evidence, especially when he needed a mechanism that would explain the origin of new variations. Another consequence of the Darwinian theory was that the historical connections between species, their genealogy, immediately suggested an explanation for the similarity between them. The more closely two species are related to each other, the more similar they will be. Homologies, those structures that were considered the same in different organisms, could now be explained as being derived from a common ancestor. The only practical problem was that the genealogical relations between species were not obvious and needed to be inferred based on the similarity between them.

Studying the development of different species offered a way to escape this circularity of reasoning. Ernst Haeckel postulated that ontogeny recapitulates phylogeny, that the developmental sequence of an individual parallels the historical sequence of evolution. For Haeckel development was simultaneously a record of history and an explanation of diversity, as new structures would occur as terminal additions in the developmental process. Development also provided a way to establish homologies; those structures that were derived from the same embryological precursors (anlagen) could be considered to be homologies and used for the reconstruction of phylogenies. Haeckel's ideas, largely discredited today, were extremely influential in the second half of the nineteenth century and led to many proposals about the shape of the "tree of life."

Entwicklungsmechanik and Developmental Genetics

The Haeckel program in evolutionary morphology, with its descriptive outlook and its tendency to speculate about phylogenetic relationships, left many younger scientists dissatisfied. They sought a mechanistic understanding of development, more in tune with the emphasis on experimentation and causal interpretation that characterized sciences like physiology or chemistry. Championed by Wilhelm Roux, this new approach to the study of development dominated late-nineteenth-century biology in Germany and the United States. In detailed and technically demanding experiments, biologists tested the influence of physical and chemical conditions such as gravitation, pressure, temperature, and varying chemical concentrations in the environment on development of select model organisms (mostly amphibians and marine invertebrates) whose free-living embryos were easy to manipulate. This new experimental program in embryology also benefited from the newly founded marine research stations. Many of these experiments were only possible in well-equipped laboratories in close proximity to the diverse biological material of the sea.

The canonical experimental styles in Entwicklungsmechanik were the destruction of certain parts of the embryo and the transplantation of specific tissues between and within embryos. Both kinds of experiments disrupted normal development and allowed researchers to discover the effects of certain parts of the embryo. Puncturing one of the two cells in a two-cell-stage frog embryo, Roux found in 1888 that only half an embryo developed. In his mosaic theory of development he then argued that during differentiation the determining factors, which are all present in the fertilized egg, are gradually distributed among the daughter cells. In a similar vein, August Weismann argued in 1892 for the separation of the germ line, which he saw as retaining the full developmental potential and being passed on through the generations, and the soma, those elements of an organism that undergo differentiation. Weismann, too, thought that an unequal distribution of hereditary material accounts for the differentiation of cells during development.

When Hans Driesch repeated Roux's experiment, shaking sea urchin embryos apart during the two-and four-cell stages, he observed the formation of complete, albeit smaller, pluteus larvae. Driesch began to think that development could not be interpreted in strictly mechanical terms. The embryos' demonstrated ability to regulate their own developmental sequence led him to argue that organisms are harmonious equipotential systems and not just complex physico-chemical machines. Organisms as individuals are instead characterized by an irreducible telos, their entelechy, that shows itself in their regulatory abilities. Driesch subsequently embraced a form of neovitalism.

The vast majority of biologists, however, did not accept Driesch's philosophizing but remained committed to experimental
study of development, mapping cell lineages and investigating fates of transplanted tissues. It was in this context that Hans Spemann and Hilde Mangold found in 1924 that a small region near the dorsal lip transplanted into the ventral side (belly) of a newt embryo could organize a second set of body axes, thus resulting in a "Siamese-twin-like" embryo. They called this specific region of the embryo the organizer, as it was capable of organizing the basic form of the full organism. In addition, researchers demonstrated that interactions between certain tissues such as mesoderm and ectoderm led to the differentiation of phenotypic structures such as the lens of a vertebrate eye, in a phenomenon called induction. The search began for the specific chemical properties of what was assumed to be the material organizer.

It was also clear that ultimately these developmentally active substances would have to be the products of heredity, since the inherited nuclear chromosomes and the genes they presumably carried, together with the material inside the egg, are what is passed on to the next generation. Research programs in developmental and physiological genetics investigated these questions and, after long and painstaking research, could identify specific causal chains, from a gene product to a phenotypic effect. Mutants, such as eye-color mutants of moths and flies, were the preferred experimental systems for this line of research. In 1940 a group headed by the biochemist Adolf Butenandt and the biologist Alfred Kühn were the first to identify and chemically characterize the substance that induced the red-eye phenotype in the moth Epestia kühniella.

After World War II, developmental biology gradually transformed itself into developmental genetics, especially after the techniques of molecular biology allowed researchers to study genes in their cellular context. One of the first genetic systems studied molecularly was the so-called lac-operon system, which regulates the expression of a lactose-digesting enzyme inside a bacterial cell. This focus on regulation continued as more and more regulatory networks of genes were found. In the context of molecular biology, development—the growth and differentiation of an organism—had been redefined as a problem of the regulation of gene expression. Aristotle's epigenesis had given way to the mechanistic preformationism of the seventeenth and eighteenth centuries and had come around again to a more sophisticated blend of preformism through heredity and epigenesis through development.

Evolutionary Developmental Biology

During the last decades of the twentieth century, evolutionary developmental biology emerged to reintegrate the two temporal processes within biology, development and evolution. Evolutionary developmental biology (Evo-Devo) is based on the recognition that all genetic changes must be expressed during development in order to produce a phenotype and thus amount to observable evolutionary changes. Development is thus the mechanism that produces the raw material of phenotypic evolution. Phenotypic evolution, in contrast, appears to be highly constrained. Of all the possible forms (the total morphospace), only a small number are actually realized. Furthermore, the diversity of life is organized in a nested hierarchy, whereby millions of species can be subsumed within a few dozen phyla, each characterized by a basic body plan (Bauplan ). In other words, the many mutational changes of genotypes are translated into a much smaller number of phenotypic variants.

In addition, discoveries since the 1980s have lent further support to the idea that the number of developmental modules (transcription factors, such as Homeobox genes and regulatory networks) is relatively small. Furthermore, these developmental modules have been conserved through millions of years during evolution, in that flatworms, insects, and mammals share a number of regulatory genes. Thus, a limited "genetic toolkit of development" produces the astonishing diversity of life. These findings have serious consequences for the age-old discussions of preformism versus epigenesis. The fact that a small number of genetic elements is responsible for the enormous diversity of life indicates that development is essentially a problem of regulation and the interaction of genetic and environmental factors. In other words, the effects of genes in development are largely context dependent. Whether a specific transcription factor turns on a gene that triggers a cascade leading to the formation of an eye or whether it establishes the gradient for differentiation of the arm, for example, depends on the specific cellular and organismal context. In addition, environmental factors, which can affect developmental plasticity, are increasingly recognized as important. The current conception of development is thus largely epigenetic, within the context of inherited material genes.

Human and Social Dimensions of Development

Interpretations of individual development have also had powerful social impacts, especially as we have learned more about human embryology and reproductive biology. For those who hold the strongest versions of the view that each individual organism begins from unformed material, the emphasis on epigenetic emergence of form suggests that investing in "nurture" will pay off. It is worth investing in parenting that requires time and energy because this can shape the developmental process. In contrast, those who accept the view that the organism has some clear defining point at which it begins as an individual, and that its form or individuality is in some important sense already set, see much less value in investing in trying to shape what develops. Development in these cases is largely a matter of playing out the intrinsic causes. The dominant version of this interpretation maintains, of course, that heredity sets the individual's differentiation and that development is really just a matter of growth.

Though no respectable scientist today would hold either of these extreme interpretations, there are strong preferences depending on whether the researcher is a genetic determinist or a proponent of developmental regulation. Historically, we can find some supporters for almost any interpretation along the range of possibilities. The public's very deeply held views about individual as well as species development make it all the more important that we have a clear understanding of the historical, philosophical, and biological contexts for developmental ideas and that we understand the social implications.

Development

Development

Development theory is largely a product of post–World War II (1939-1945) thinking in the social sciences and international policy studies. The key intellectual challenges for development theory are these: What are the causes of economic transformation in human societies? What are some of the policies through which governments can stimulate the processes of economic growth? These questions have been the subject of inquiry within classical political economy for several centuries, and interest in the determinants of growth and modernization has been part of economic theory since its beginnings. But modern development theory took its impulse from global developments following World War II—the needs of reconstruction of Europe and Japan following World War II; the creation of international monetary and trading regimes to facilitate international economic interaction; the circumstances that followed from the dissolution of European colonies in Asia, Africa, and Latin America; growing attention to the persistence of poverty in the developing world; and focus in the 1990s on the phenomena of globalization.

The concept of development has encompassed several separate ideas in the past sixty years: the idea of modernization of economic and social institutions, the idea of sustained economic growth within a national economy, the idea of the continuing improvement of the material well-being of the earth’s human population, the idea of more extensive utilization of the world’s resources, and the idea of the replacement of “traditional” institutions and values with “modern” successors. Some of the large questions that have guided development theory include these: What are the features of society that can be characterized as “modern”? What causes a society to undergo sustained “modernization” and sustained economic growth? What institutional features are important causes in economic development? What steps can governments or other major institutions take to stimulate development? What is the significance of the specific features of western European economic development since 1600? Are there alternative pathways through which “modernization,” growth, and improvement of human well-being can occur? Are there cultural assumptions that are made in valorizing development, growth, and modernization over tradition, moderate consumption, and stable cultural practices? How can one best define the goals of development in terms of human well-being? How should considerations having to do with equality, equity, and justice be incorporated into analysis and policy of development?

Development theory has taken shape through efforts in several areas of the social sciences: economics (theories of efficient markets, trade, and income distribution); sociology (research on concrete processes of social change in different parts of the world); anthropology (research on the values and practices of a range of non-Western cultures); political science (research on the institutions and interests that drive international economic policy); history (research on the dynamic circumstances that created modern national and international economic institutions); and critical social science (focus on features of inequality, power, and exploitation that have often characterized international economic institutions). Each of these strands captures something important about the historical experience of parts of the modern world, and yet they fall short of a full and general approach to the topic of development. Development theory is an expansive, eclectic, and interdisciplinary field defined by a diverse set of questions and methods—not an exact subdiscipline within economics.

Decolonization and the aftermath of World War II stimulated a wave of academic and policy interest in the dynamics of economic growth and development. President Harry Truman highlighted the crucial importance of addressing global issues of poverty and hunger in his 1949 State of the Union address, an emphasis that stimulated new United States and international commitments in support of economic development in the decolonized world. The 1950s witnessed a surge of early development theory, in the hands of such authors as Simon Kuznets, W. Arthur Lewis, and Ben Hoflitz. A central thrust of these efforts was the formulation of economic theories of growth that, it was hoped, could help to guide policy in the economic transformations associated with decolonization. Postwar development theory also provided some of the intellectual foundations for the establishment of postwar international economic institutions such as the World Bank and the International Monetary Fund. Much of this work presupposed the idea that there were distinct stages of economic development, and it focused on the relationship between economic growth and savings as the basis for capital formation. The role of trade in economic development also played a central role in these theories. W. Arthur Lewis’s theory of the “dual economy” was particularly prominent as a basis for attempting to understand the economies of the previously colonial world. This model postulates an economy consisting of a “traditional” sector (labor) and a “modern” sector (capital). Lewis postulated that firm owners in the modern sector were profit-maximizing, whereas those in the traditional sector were not, and that there was surplus labor in the traditional sector. So a strategy for growth is to induce a shift of economic activity from the traditional to the modern sector. This approach would increase savings and capital formation, leading to growth and rising incomes. Other economists such as Irma Adelman and Hollis Chenery cast doubt on the stage theory and further broadened the perspective by bringing distribution and welfare into the discussions.

There has been emphasis since the 1960s—sometimes waxing, sometimes waning—on the crucial importance of alleviating poverty in the developing world. Throughout much of its history the World Bank has expressed its adherence to the priority of poverty alleviation. The United Nations Millennium Goals for 2005 place poverty alleviation at the center of the development agenda for the coming fifty years. But even placing a sincere priority on poverty alleviation, there is a wide range of disagreement over the steps that should be taken to achieve this goal.

Several important frameworks of thought have been important in development theory thinking. Neoliberal development theory reflects the folk wisdom of neoclassical economic and political theory. Described as the “Washington Consensus,” this approach to development postulates that modern economic development requires free markets, effective systems of law, and highly limited powers of government. The slogan of “Getting the Prices Right” was a rule of thumb for economic institutional reform in countries receiving advice and assistance from international institutions. This school of thought places great importance on free trade within the international economic system. Neoliberal structural adjustment reforms in the 1980s, enforced through International Monetary Fund and World Bank policies, pushed third-world governments toward harsh domestic reforms (currency devaluation, reduction of programs aimed at the poor, elimination of subsidies for rural development, liberalization of trade practices). Critics have argued that these structural adjustment policies have had the effect of further impoverishing the poorest of many developing societies.

Critical of the neoliberal consensus is an influential group of development theorists who emphasize the centrality of human well-being in development theorizing and the crucial role that public policies and expenditures play in successful efforts to improve the well-being of the poor in developing societies. Amartya Sen, Martha Nussbaum, and others argue for placing a nuanced theory of human development grounded in capabilities and functionings at the center of development policy. And they argue for the crucial role that public policy has in creating the human welfare infrastructure that is essential for the successful alleviation of destitution: public health, nutrition, free education, and democratic freedoms. On this view, the narrow conception of the role of the state associated with the neoliberal approach almost inevitably implies further degradation of the conditions of life of the least-well-off in the developing world. A concrete achievement of this approach is the creation and maintenance of the Human Development Index by the United Nations Development Program. This index is designed to provide a measure of economic development that goes beyond measuring growth of per-capita income, and instead focuses on measures that are correlated with quality of life: health, longevity, and educational attainment, for example. Another such measure is the Physical Quality of Life Index.

There have been critical voices within development theory throughout its history. Postwar theories of colonialism emphasize the extractive role that the system of colonial control represented, with a flow of natural resources from periphery to metropole. Dependency theory is the view that the world economy since 1945 has been constructed around a set of institutions that systematically disadvantage the South for the benefit of the North, by structuring production and trade in such a way as to limit economic growth in the third world. The South is integrated into the “modern world system,” but on terms that systemically work to the disadvantage of the countries and peoples of the periphery. The Brandt Report (Independent Commission on International Development Issues, 1980) focused attention on the relations between the wealthy North and the impoverished South. This approach emphasizes structural inequalities, systemic institutional disadvantages, patterns of unequal exchange, and resulting uneven development. A different line of critical thought emerges from cultural critics of modernization. Arturo Escobar is a central voice in this body of criticism. This perspective offers a critique of the discourse and presuppositions of development thinking in the West: the presumed primacy of Western values, the unquestioned importance of consumerism, the teleology associated with the concept of “modernization,” and other ways in which the values and assumptions of development theory reflect unquestioned ethnocentrism and universalism.

The concept of development incorporates several debatable assumptions. First, it has a tendency towards Eurocentricism. The paradigm of development incorporated in much development theorizing is the experience of western Europe during the Industrial Revolution. Non-European societies that undergo “development” (Japan, Taiwan, and Argentina) are frequently categorized in terms of a baseline comparison to the western European experience. Historical research conducted since the mid-1980s casts doubt on this single-track theory. Asian economic development in the seventeenth century and the twentieth century gives substantial evidence of the availability of alternative pathways of development, and there is considerable institutional variation within regions of western Europe itself.

Second, the concept of development is burdened with an implicit teleology. The word implies a progressive transformation from “less developed” to “more developed”; it implies the creation of more complex, sophisticated, and humanly adequate systems out of simpler and less adequate systems. (Rostow’s concept of “stages of growth” captures this assumption precisely.) Historians have long since abandoned the idea that history has directionality, and have demonstrated the many false starts, wrong turns, reversals, and stalls that all societies have experienced.

Third, the concept of development brings with it an idealized set of assumptions in the background about what a “developed” society ought to include: consumerism, democracy, markets for everything, individualism, impersonal legal systems, large complex societies, and a high material standard of living. This is a social ideal that is deeply embedded in development theory and that can be legitimately questioned. The values that are associated with western consumer culture represent one possible framework of human values—but only one such framework. In fact, it may be that the constraints of long-term environmental sustainability make this complex of values doubly questionable.

Finally, development theory has been forced to confront the contradictions between economic growth and environmental sustainability. Resource depletion, destruction of forests and wetlands, urban sprawl, air and water pollution, and global climate change resulting from carbon dioxide emissions all call into doubt the feasibility of permanent economic growth. Prudent multigenerational planning for the future of the planet will require more consistency between the needs of consumption and the needs of environmental sustainability.

Independent Commission on International Development Issues. 1980. North-South, a Program for Survival: Report of the Independent Commission on International Development Issues. Cambridge, MA: MIT Press.

Morris, Morris David. 1979. Measuring the Condition of the World’s Poor: The Physical Quality of Life Index, Pergamon Policy Studies. New York: Published for the Overseas Development Council by Pergamon Press.

Development

Biology
COPYRIGHT 2002 The Gale Group Inc.

Development

Reproduction and development are integral factors of life. Multicellular organisms arise through a process that begins with the fertilized egg and ends with a new individual. The fertilized egg undergoes cell divisions to increase the number of cells; simultaneously, the cells produced differentiate into the organs and organs systems of the fully formed organism. The scientific study of these developmental processes is called embryology. Aristotle (384–322 b.c.e.), considered the first embryologist, described the growth of a chick embryo from a small dot of tissue to a fully formed bird.

Early Ideas in Embryology

Prior to the mid-1800s, scientists believed that development was the result of preformation. Preformation means that animals develop from an already existing miniature animal that merely required the right conditions to unfold and grow into a new organism. Scientific debates over whether the miniature animal was contained in the egg (ovum) or the sperm raged for decades. Scientists who believed that the miniature animal was in the egg were called ovists; those who believed that the miniature animal was in the sperm were called spermists.

In 1675, Marcello Malpighi (1628–1694), an ovist, reported seeing a miniature chick in the chicken egg. Anton Leeuwenhoek (1632–1723), a spermist, reported seeing a miniature human (homunculus) in each sperm (see Figure 1). In 1775, Lazzaro Spallanzani (1729–1799) demonstrated that both egg and seminal fluid were needed to produce a new individual. He conducted a series of experiments using amphibian eggs and seminal fluid. When the eggs were exposed to seminal fluid, they began to develop. However, if the eggs were exposed to filtered seminal fluid, fewer eggs developed. The more highly the seminal fluid was filtered, the fewer eggs developed. If the eggs were combined with the material left on the filter paper, they began to develop. Although Spallanzani correctly concluded that both egg and seminal fluid were necessary for development, he believed that the sperm seen in the seminal fluid were parasites . He postulated that the fertilizing agent was composed of the proteins and fats in the fluid.

William Harvey (1578–1657) viewed embryological development as a continuing process of remodeling and growth from unspecialized tissues to specialized structures. This theory, which is termed epigenesis, was largely ignored until 1759 when Kaspar Friedrich Wolff (1733–1794) offered empirical evidence of epigenesis from his detailed studies of chick development. In 1828, Karl von Baer (1792–1876) described four fundamental concepts of development. These four concepts, referred to as von Baer's law, are: (1) general features such as appendages appear earlier in the embryo than specialized features such as fingers; (2) development proceeds from general to specific characteristics; (3) as an embryo develops, it becomes increasingly different from other species; and (4) the early embryo of a higher animal is never like an adult of a lower animal, but only similar to the lower animal's embryo. Because they delineated the basic concepts of development, Wolff and von Baer are considered founders of modern embryology.

Fertilization

The process of development begins with the fusion of gametes : egg (ovum) and sperm. The motile sperm swims to the egg, pierces its cell membrane and enters the cell. Fertilization is the fusion of the nuclei of the egg and sperm, and the single cell that results from this fusion is called the fertilized egg or zygote (see Figure 2). During fertilization, the genetic material of the sperm and egg are combined. Each gamete is haploid , that is, it contains one-half of the normal number of chromosomes for the species. At fertilization, the gametes combine to produce a zygote with the full number of chromosomes for that particular species. Another way to say this is that fertilization restores the diploid number. For example, haploid human gametes have twenty-three chromosomes; when the egg and sperm fuse, the diploid state of forty-six chromosomes is restored to the zygote. Thus, each parent contributes one-half of the chromosomal complement of the new individual, resulting in a new organism with genetic characteristics of both parents. This single cell, the fertilized egg, gives rise to all the organs of the individual—muscles, brain, liver, eyes—through highly regulated and timed processes.

The stages from fertilization to the birth or hatching of an individual organism are identical to those of all individuals of the same species. Developmental stages include rapid cell division or cleavage; formation of the germ layers through the process of gastrulation; and differentiation and growth of the organs and organ systems. These stages are categorized as the periods of embryogenesis (cleavage and gastrulation) and organogenesis (formation of organs and organ systems).

Cleavage

As soon as the sperm enters the egg, the cell membrane of the egg undergoes changes that prevent the entrance of additional sperm. Meanwhile, the chromosomes from each parent come together and, within a few hours, the first cell division begins. The egg degrades the cytoplasm and organelles of the sperm; only the chromosomes of the sperm contribute to the fertilized egg.

The early cell divisions of the fertilized egg are called cleavage. The fertilized egg divides into two daughter cells called blastomeres. These two
blastomeres divide into four blastomeres, the four blastomeres divide into eight, and so on. During cleavage, the total number of cells increases, but the size of each cell decreases. The reason for this strange situation is that cell division occurs so rapidly that there is not enough time for the individual cells to grow bigger. The constant doubling of cells during cleavage results in a multicellular embryo very quickly.

In a short period, the embryo has over one hundred cells arranged as a solid ball of blastomeres called a morula. The cells of the morula rearrange themselves into a single layer of cells surrounding a fluid-filled central cavity; the embryo at this stage is called a blastula (see Figure 2).

Gastrulation

The next step in development is the formation of the gastrula by invagination, the folding in of the cells of the blastula at a point called the blastopore. The resulting gastrula is a double-layer cup of cells. The outer layer of cells is termed the ectoderm and the inner layer of cells is termed the endoderm. The inner endodermal layer surrounds a new cavity, the primitive gut. A third layer of cells, the mesoderm, develops between the ectoderm and endoderm in most animals. Ectoderm, mesoderm and endoderm are the three germ layers from which all cells, tissues and organs develop (see Figure 2).

Induction

One of the more fascinating aspects of development is the determination of body form, pattern, and differentiation. Put simply, how does a cell know what it is supposed to grow up to be? How do cells of the endoderm know they are supposed to form the digestive and respiratory systems? Induction is the process during which individual cells are "told" what they are supposed to become. A modern understanding of molecular events in development is discussed in the article Genetic Control of Development in volume 2 of this reference work. This essay outlines some pioneering work by Hans Spemann and Hilde Mangold.

Hans Spemann (1869–1941) received the Nobel Prize in 1935 for over twenty years of research on development in amphibians. In a series of elegant and delicate "baby hair loop" experiments, he demonstrated that when cells invaginate during gastrulation, they are induced to form specific cells and organs and that the primary inducer is a specific region of the blastopore. Spemann tied a strand of baby's hair around a fertilized newt egg so that the nucleus and some cytoplasm were on one side of the ligature while the other side contained only cytoplasm. After several cell divisions, Spemann loosened the ligature and allowed a nucleus to pass over into the other side. When cell divisions began on the side with the transported nucleus, the ligature was again tightened to separate the two masses of cells. The result was the production of two newt larvae, one a bit older than the other.
These experiments demonstrated that all the nuclei of an early embryo are capable of producing embryos. This ability is the basis for much current practice of cloning agriculture or lab animals.

In later experiments, Spemann found that the location of the ligature was important. If the ligature were placed so that each half of the fertilized egg contained the certain cells (called the gray crescent because of their color) from the region destined to become the blastopore, two newts would develop. However, if the ligature were placed so that the gray crescent was only on one half of the cell, that part would form a newt, but the half without the gray crescent would remain a formless mass of cells he called the belly piece. Further experimentation demonstrated that during gastrulation cells became committed to their developmental fates.

Spemann and his graduate student Hilde Mangold (1898–1924) demonstrated that specific cells of the blastopore are the only determining region
in the gastrula. When these cells were transplanted to other embryos, the embryos were induced to undergo gastrulation and form a second embryo. These experiments that Mangold performed for her doctoral dissertation formed the foundation for much of Spemann's later work. Mangold died at age twenty-six when the heater in her apartment exploded. She could not share the Nobel Prize awarded to Spemann eleven years later because the prize is not awarded posthumously.

Protostomes and Deuterostomes

The fate of the blastopore is used to classify animals that have three germ layers into two large categories, protostomes and deuterostomes . Most adult animals have two external openings, the mouth and the anus, into the digestive tract. During gastrulation, the blastopore is the opening into the primitive gut. During further development, the blastopore becomes the mouth in animals classified as protostomes; the blastopore becomes the anus in deuterostomes. Organisms belonging to the phyla Mollusca (clams and snails), Arthropoda (insects and crustaceans) and Annelida (earthworms) are protostomes; members of the phyla Echinodermata (starfish) and Chordata (fish and humans) are deuterostomes. The type of cleavage and the development of the body cavity are other important differences between the protostomes and deuterostomes.

Protostomes exhibit spiral cleavage in which the blastomeres divide at acute angles to one another and are not aligned over one another. If one of these blastomeres is removed from the embryo, neither the removed blastomere nor the remaining cells develop into an individual. This type of determinate cleavage indicates that the fate of the daughter cells is determined early in development. A final characteristic of protostomes is that the body cavity, or coelom, develops as a split within the middle of the mesodermal layer. This type of coelom formation is termed schizocoelous development.

Deuterostomes, on the other hand, exhibit radial cleavage in which the blastomeres divide perpendicular or parallel to one another and are
aligned over one another. If one of these blastomeres is removed from the embryo, both the removed blastomere and the remaining cells can develop into individual organisms. This type of indeterminate cleavage indicates that the fate of the daughter cells is determined later in development. Identical twins are possible because of indeterminate cleavage. Genetic testing of human embryos is possible because of indeterminate cleavage: One blastomere can be removed from an eight-celled embryo and tested without interfering with the normal development of the remaining seven cells. In deuterostomes the coelom develops as buds from the primitive gut. This type of coelom formation is termed enterocoelous development.

Biogenetic Law

Ernst Haeckel (1834–1919), a physician, was so influenced by Charles Darwin's The Origin of Species that he gave up medicine and devoted himself to comparative anatomy. He disagreed with Darwin's theory of natural selection, and suggested that the environment acted directly on organisms, producing new species. In 1868, he proposed the biogenetic law, which sought to explain evolution as a series of stages in which the new characteristics of the next animal to evolve are simply added on to the lower animal. Briefly put, his biogenetic law stated that ontogeny recapitulates phylogeny (the
embryological development of a particular species repeats the evolutionary history of that species). Although Haeckel is best remembered for his "ontogeny recapitulates phylogeny" statement, he also coined the terms phylum , ecology, and phylogeny.

Modern scientists do not subscribe to the biogenetic law as postulated by Haeckel. However, there are elements of recapitulation that are important in comparative embryology. In 1828, Karl von Baer pointed out that vertebrates share common characteristics during development (see Fig. 3). Examination of vertebrate embryos reveals that during corresponding stages of early development, the embryos appear to be very similar. For example, all vertebrate embryos pass through stages in which they have gill pouches. The pouches eventually develop into the gill apparatus in fish; in later-evolving vertebrates that do not have gills, the gill pouches undergo further refinement and develop into structures associated with the head and neck. Similarly, all early vertebrate embryos have
tails, which persist in some animals but regress during the later stages of development of humans. Thus, the individual development of an animal occurs through a series of stages that paint a broad picture of the evolutionary stages (phylogeny) of the species to which it belongs.

development

de·vel·op·ment
/ diˈveləpmənt/
•
n.
1.
the process of developing or being developed:
she traces the development of the novel
| the development of less invasive treatment. ∎
a specified state of growth or advancement:
the wings attain their full development several hours after birth. ∎
a new and refined product or idea:
the latest developments in information technology. ∎
an event constituting a new stage in a changing situation:
I don't think there have been any new developments since yesterday. ∎
the process of converting land to a new purpose by constructing buildings or making use of its resources:
land suitable for development. ∎
an area of land with new buildings on it:
a major housing development in Chicago. ∎ Chess
the process of bringing one's pieces into play in the opening phase of a game.
2.
the process of starting to experience or suffer from an ailment or feeling:
the development of brittle bones.3.
the process of treating photographic film with chemicals to make a visible image.

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development

development The complex process of growth and maturation that occurs in living organisms. Cell division and differentiation are important processes in development. In vertebrate animals there are three developmental stages: (1) cleavage, in which the zygote divides to form a ball of cells, the blastula; (2) gastrulation, in which the cells become arranged in three primary germ layers (see gastrula); (3) organogenesis (or organogeny), in which further cell division and differentiation results in the formation of organs. The development of many invertebrates (e.g. insects) and amphibians involves the process of metamorphosis. In all organisms, development is directed by the coordinated expression of various genes in time and space, regulated by the complex interplay of the gene products (see morphogen). Early development may be influenced by maternal genes (see maternal effect genes) as well as embryonic genes. Genes involved in development are often highly conserved across very diverse organisms, as exemplified by the Hox genes that determine the structures along the head-to-tail axis of most animals. See also homeotic genes; morphogenesis; primary growth.

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development

development (also called free fantasia, or working-out. Fr. développement; Ger. Durchführung, i.e. ‘through-leading’; It. svolgimento, i.e. ‘unfolding’). The treatment of the detailed phrases and motifs of a previously heard theme (‘subject’) in such a way as to make new passages, often of a modulatory nature. The second section of sonata form, coming between exposition and recapitulation, is the development. With the expansion of the symphony, the development section became increasingly complex and important. Beethoven departs from convention in his 3rd sym. by introducing new thematic material in this section. There is also a development in fugue.

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